Hepatitis D is a disease caused by infection with a small, circular enveloped RNA virus known as hepatitis delta virus (HDV). HDV was first discovered in 1977 (Rizzetto et al., Gut 1977; 18: 997-1003) and was later shown to be the infectious agent of a new form of hepatitis (Rizzetto et al., J Infect Dis 1980; 141: 590-602; Rizzetto et al., Proc Natl Acad Sci USA 1980; 77: 6124-8; Wang et al., Nature 1986; 323: 508-14; Mason et al., In: Fauquet C M, Mayo M A, Maniloff J, Desselberger U, Ball L A, eds. Eight Report of the International Committee on Taxonomy of Viruses. London: Elsevier/Academic Press, 2005; 735-8). It has been recently estimated that 15-20 million people are infected with HDV, which requires concurrent infection with hepatitis B virus (HBV) for its life cycle, although this number may be underrepresented due in part to the lack of systematic screening for HDV infection in HBV-infected individuals (Pascarella and Negro, Liver International 2011, 31: 7-21).
Hepatitis D viruses are spherical particles that contain a core structure formed from an HDV genomic RNA that is complexed with about 70 molecules of HDAg (in both small and large forms) (Ryu et al., J Virol 1993; 67:3281-7). HDV is believed to enter the cell using the same receptor as HBV, utilizing the HBV envelope proteins as its outer coat. The envelope is comprised of approximately 100 copies of HBV surface antigen proteins (small, middle and large HBsAg). The large HDAg and HBsAg are sufficient to form virus particles, which are not infectious unless HDV RNA is also included, and small HDAg increases the packaging efficiency of the virus (Chen et al., J Virol 1992; 66: 2853-9; Wang et al., J Virol 1994; 68: 6363-71).
Once inside the cell, HDV uses host cellular RNA polymerases. Three RNAs accumulate during virus replication processes. The HDV genome is a circular negative single-stranded RNA of about 1672-1697 nucleotides (Radjef et al., J Virol 2004; 78:2537-44) and contains a ribozyme domain, spanning nucleotides 680-780, and a putative promoter site for HDAg RNA (Beard et al., J Virol 1996; 70: 4986-95). The antigenome, which contains the open reading frame coding for HDAg and a ribozyme domain (Sharmeen et al., J Virol 1988; 62: 2674-9; Ferre-D'amare et al., Nature 1998; 395:567-74) is the perfect complement of the genome and its replication occurs through RNA-directed RNA synthesis without any DNA intermediates (Chen et al., Proc Natl Acad Sci USA 1986; 83: 8774-8). The mRNA directs the synthesis of HDAg.
There is only one known protein encoded by the HDV genome, and it consists of two forms, a 27 kDa large (L) HDAg (HDAg-L or L-HDAg) (214 amino acids) and a 24 kDa small (S) HDAg (HDAg-S or S-HDAg) (195 amino acids). The proteins differ by about 19 amino acids at the C-terminus of the large HDAg. The N-terminus of the HDV antigen is responsible for nuclear localization signaling, the middle domain of HDV antigen is responsible for RNA binding, and the C-terminus is involved in virion assembly and inhibition of RNA assembly. HDAg-S is produced in early stages of the viral infection and supports viral replication. HDAg-L is produced later in viral infection, inhibits viral infection, and is required for assembly of viral particles.
HDV infection occurs only in individuals who are co-infected with a different virus, hepatitis B virus (HBV), and more specifically, only in HBV surface antigen (HBsAg)-positive individuals. As discussed above, HDV requires the HBV HBsAg for particle formation and transmission, and so HBV is essential for HDV virion assembly and release. There are two primary known ways in which HDV infects an individual. In the first, called co-infection, HDV and HBV can simultaneously co-infect an individual as an acute infection, and this type of infection results in about 95% recovery of most persons, similar to recovery rates for acute HBV infection alone. The second type of HDV infection, which is more common, is “superinfection”, where HDV acutely infects an individual who already has chronic HBV infection. In this case, the HDV infection progresses to chronic HDV infection in about 80-90% of individuals, and it is this chronic HDV infection that is the more severe form of the disease. Therefore, superinfection can be classified as the earlier acute HDV superinfection of a chronic HBV carrier or the later HDV chronic infection. A third, but controversial, form of potential HDV infection, called helper-independent latent infection, was initially reported in 1991, and was described as occurring during liver transplantation (Ottobrelli et al., Gastroenterology 1991; 101: 1649-55). In this form of infection, a patient's hepatocytes might be infected with HDV alone (e.g., during liver transplantation when HBV transmission is prevented by administration of hepatitis B immunoglobulins), but if residual HBV escapes neutralization, or the patient is otherwise exposed to HBV subsequently, the HDV infected cells may be “rescued”. This form has been demonstrated in animal models, but such infection of human hepatocytes remains controversial.
Chronic hepatitis D is now considered to be the most severe form of viral hepatitis in humans (for a detailed review of the disease and associated HDV, see Grabowski and Wedemeyer, 2010, Dig. Disease 28:133-138; or Pascarella and Negro, Liver International 2011, 31: 7-21). Individuals chronically infected with HDV have an accelerated progression to fibrosis, increased risk of hepatocellular carcinoma, and early decompensation in the setting of cirrhosis. The disease may be asymptomatic or present with non-specific symptoms, and the diagnosis may only occur once complications appear at the cirrhosis stage of the disease. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels are persistently elevated in most patients and can be used to monitor the disease. Within 5-10 years, as many as 70-80% of chronic hepatitis D patients may develop cirrhosis (Rizzetto et al., Ann Intern Med 1983; 98: 437-41; Govindarajan et al., Hepatology 1986; 6: 640-4) and 15% within 1-2 years (Saracco et al., J Hepatol 1987; 5: 274-81). HDV infection may also accelerate the development of hepatocellular carcinoma (HCC).
HDV infection is most prevalent in the Mediterranean basin, the Middle East, Central and Northern Asia, West and Central Africa, the Amazonian basin, Venezuela, Colombia and certain islands of the Pacific, although the virus is present and/or emerging worldwide (e.g., Russia, Northern India, Southern Albania, mainland China, and some Pacific Islands). HDV infection is parenterally transmitted, most typically through drug use or exposure to blood or blood products. Sexual transmission of HDV is less common, and perinatal transmission of the virus is rare.
Regardless of the mode of HDV infection, there is currently no good option for the treatment or prevention of HDV infection. The anti-viral drugs used to treat other viruses that infect hepatocytes (e.g., antiviral drugs for HBV or HCV), are not effective against HDV. Immunomodulatory drugs such as corticosteroids or lemivasole have not been effective (Rizzetto et al., Ann Intern Med 1983; 98: 437-41; Arrigoni et al., Ann Intern Med 1983; 98:1024), nor have thymus-derived peptides (Rosina et al., Dig Liver Dis 2002; 34: 285-9; Zavaglia et al., J Clin Gastroenterol 1996; 23: 162-3). This leaves interferon treatment (e.g., pegylated interferon-α; pegIFN-α) as the only presently approved treatment for HDV infection. However, it is known that HDV can interfere with IFN-α signaling in vitro and indeed, treatment of HDV with pegIFN-α suffers from treatment failures and low response rates. In one prospective trial, only 21% of the patients achieved HDV RNA negativity and only 26% had a biochemical response (Niro et al., Hepatology 2006; 44:713-20), and similar results have been obtained in other trials, where sustained response to therapy (cure) remains very low. Therefore, there is a need in the art for new prophylactic and therapeutic approaches for HDV infection.